The present invention relates to the production of combustible gas or the like (e.g., synthetic compositions similar to natural gases usable as an industrial reactant or nonburning gas or as a combustant). In the prior art, most coal gasification processes rely on the reaction of coal or char with steam to generate the necessary hydrogen in a reaction essentially as follows: EQU C+H.sub.2 O(g).fwdarw.CO+H.sub.2 .DELTA.H.sub.R (1200.degree. K.)=32.457.degree. K. cal/g mole I.
The equilibrium constant of the above reaction becomes larger than 1 (K.sub.p &gt;1) only when the temperature exceeds 947.degree. K. The reaction is highly endothermic and its rate becomes fast enough to be practical only above 1220.degree. K. The high energy requirement at such high temperatures can be met, as practiced in most coal gasification processes, by partial combustion of coal (or char) using high-purity oxygen.
The heat may be supplied by external heat exchangers, but only at the disadvantage of low throughput due to the limited rate at which heat can be supplied through the walls to the reactants. In addition, the very high operating temperature will require (at a prohibitive cost) exotic materials in the construction of the gasification vessels. Furthermore, the fusion of ash around 1400.degree.-1500.degree. K. can seriously hinder a successful operation of such heat exchangers which must be operated above 1400.degree. K. to obtain practical gasification rates. Heat supply by recirculating melt or inert solids which can be heated in a vessel separate from the gasification vessel can substantially increase the heat transfer rates, but still retains many of the problems associated with very high temperature operations.
The heat for the steam-carbon reaction can be also supplied internally from the reaction of CaO with CO.sub.2 which is one of the gasification products. Simultaneous removal of undesired CO.sub.2 as CaCO.sub.3 further encourages the formation of H.sub.2. Although this process is conceptually ideal, it poses many technical problems inherent to gas-solid reactions at high temperature.
It is therefore a principal object of the invention to provide a coal gasification process with reduced energy requirements and/or reduced oxygen requirements compared to the prior art processes.
It is a related object of the invention to provide such gasification process operating at generally lower temperatures and a wide range of pressure compared to the prior art processes.
It is a further object of the invention to utilize heat available in reaction steps and/or as sensible heat in one or more of the reaction products to provide all or a portion of the input energy requirement of the process, consistent with one or more of the preceding objects.
It is a further object of the present invention to provide such a process which can accept lower cost coal, particularly high sulfur/coal.